The present invention relates to shock sensors, and more particularly to shock sensors using conductive liquids to complete an electric circuit.
A shock sensor is disclosed in Bitko U.S. Pat. No. 4,683,355 issued to the present inventor on Jul. 28, 1987. One embodiment of that shock sensor is depicted in the accompanying FIG. 1. That shock sensor 10 comprises a housing having a first housing part 12 to the outside of which is connected a first contact terminal 14. The first housing part 12 comprises a non-mercury-wettable, electrically conductive material, such as steel, and is capable of conducting a charge from the terminal 14 to the interior of the first housing part 12.
A mercury-wettable, electrically conductive insert 16 is mounted in electrically conductive engagement with the interior of the housing part 12. Preferred materials for the insert 16 include nickel-copper or nickel-platinum alloys. A thick layer of mercury 18 is placed in contact with a flat mercury-wettable surface of the insert 16.
A second housing part 20 is attached to the first housing part 12. The part 20 comprises a non-mercury-wettable, electrically conductive material, such as steel, and has a central bore therethrough. A nonconductive, non-mercury-wettable core 22, such as glass or an elastomeric material is provided in the bore so as to support a second contact terminal 24 therein. The second contact terminal 24 is arranged such that one end thereof projects into the housing and one end projects out of the housing.
The second housing part 20 is attached to the first housing part 12 by welding or other suitable means. The entire housing is hermetically sealed, and the mercury layer 18 is disposed directly across from the second contact terminal 24. When the shock sensor 10 is at rest, the surface tension of the mercury layer 18 holds the mercury together such that a gap is created between the mercury 18 and the second contact terminal 24. A gas compatible with the mercury 18, such as an inert gas or hydrogen, is provided within the hermetically sealed housing.
The surface tension of a liquid is a force that causes a droplet of the liquid to assume a spherical shape in a zero gravity field. However, the shape and position of a body of liquid which is at rest under conditions of constant pressure and temperature, depend upon the equilibrium of three forces. These forces are (1) the surface tension of the liquid, (2) the magnitude and direction of all forces, including gravity, acting on the liquid, and (3) the degree of wetting between the liquid and any solid surface in contact with the liquid.
The surface tension of mercury is relatively high compared to the surface tension of many other liquids. The relatively high surface tension allows a large quantity of mercury to adhere to a surface to which it is wetted. Accordingly, it will be appreciated that gravitational forces combine with surface tension to cause the mercury 18 to collect toward the center of the insert 16, which is arranged directly below the second contact terminal 24.
A surface is considered to be wetted with a liquid if the liquid forms a low contact angle with the surface. A small quantity of liquid on a non-wetted surface will tend to bead, while on a wetted surface, the liquid will tend to spread itself uniformly over the wetted surface. In addition, as a result of an impact, the liquid will leave a non-wetted surface without generating any restoring surface tension forces, whereas resisting forces will be present in the case of a liquid wetted to a surface.
It will be appreciated that if the shock sensor 10 is subjected to a longitudinally directed shock, such as for example a sudden impact or abrupt deceleration, the mercury 18 will be subjected to a longitudinal force, e.g., force A in FIG. 1, and thus will be displaced toward the contact 24. The combined effects of: Force A, gravity, and the surface tension of the mercury cause the mercury to redistribute and protrude from the insert 16. If the Force A is greater than a predetermined value, the mercury 18 will protrude sufficiently to contact the second contact terminal 24.
When the mercury 18 contacts the tip of the second contact terminal 24, the surface tension of the contacting mercury 18 will cause the mercury 18 to stay temporarily in electrical contact with the second contact terminal 24, even though the center of gravity of the mercury 18 may tend to oscillate for a brief period, thus avoiding contact bounce problems. When the mercury 18 contacts the second contact terminal 24, a circuit is completed between the first and second contact terminals 14, 24 by means of the housing 12, the insert 16, and the mercury 18.
After the shock is over, the surface tension of the mercury 18 restores the mercury 18 back to its original configuration, thus breaking the circuit between the contact terminals 14, 24.
On the other hand, if the shock is directed sideways, i.e., toward the left or right hand side in FIG. 1, then it is necessary for the mercury to rebound off the side surface 13 of the container in order to reach the contact terminal 24. In order for that to occur, it is necessary that the minimal sideways force for closing the switch be stronger than the minimal longitudinal force A for closing the switch. This is also true in the case of a Force A' directed at an acute angle relative to Force A, because the mercury may be displaced somewhat laterally relative to the contact terminal and thus fail to make contact therewith unless it rebounds off the side surface 13. Hence, to provide a sensing system which is multi-directionally sensitive for a particular shock level, it would be necessary to provide a large multitude of sensors.
Also, the sensor 1 depicted in FIG. 1 forms a cavity 15 in which mercury can become caught and separated from the main mercury volume, thereby adversely affecting the behavior of the sensor.
Moreover, for the reasons discussed above, the FIG. 1 shock sensor would not be very sensitive to centrifugal force caused by rotation of the sensor along its longitudinal axis, i.e., an axis coinciding with Force A in FIG. 1.
Therefore, it would be desirable to provide a shock sensor which is sensitive in a wider variety of directions to a given shock value, and also to provide such a shock sensor which is more sensitive to centrifugal force.
From the foregoing description, it will be appreciated that the sensor 10 is most sensitive to shocks applied in the direction of Force A, since the shock magnitude required to close the switch is smallest in that direction. It would further be desirable to increase the sensitivity of the sensor so that it will close in response to weaker shocks than will the sensor 10 of FIG. 1.